Optical sensitizer device and method for low-energy laser ignition of propellants

ABSTRACT

An igniter system and method for igniting a propellant by an optical energy source. The igniter system including an igniter composite and a propellant. The igniter composite including a reactive component and a fluoropolymer. The propellant is coupled to the igniter composite. The igniter composite has a composition including nano-aluminum at ideal stoichiometry in polyvinylidene fluoride. The igniter composite is configured to achieve a sustained ignition from a wavelength emitted from the optical energy source. The wavelength is between around 250 nanometers to around 1100 nanometers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 63/182,017, filed on Apr. 30, 2021. The entire disclosure of theabove application is hereby incorporated herein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under FA9550-19-1-0008awarded by the Air Force Office of Scientific Research (AFOSR) and underDGE-1333468 awarded by the National Science Foundation Graduate ResearchFellowship Program. The government has certain rights in the invention.

FIELD

The present disclosure relates to igniter systems and, moreparticularly, to propulsion igniter systems.

INTRODUCTION

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Solid propellants are employed in a wide range of applications from theinflation of airbags to propulsion systems for rockets. The ignition ofsolid propellants must be carefully controlled and modified on a per-usebasis due to the specific ignition requirements of each application.Pyrotechnics, propulsion, and munitions all have unique requirements forsafe and reliable ignition. Although each application may have specificrequirements, the propellant should have a high density, a smallignition delay, and consistent and controlled ignition. Optical ignitionis of interest as it can potentially reduce the input energy needed forignition while providing more spatial and temporal control and improvedsafety by eliminating the electrical systems used in pyrogens and bridgewires, thereby militating against stray electrical charges.

Optical ignition can be classified into two categories: broadbandirradiation, characteristic of flash ignition, and coherent, singlewavelength energy, characteristic of laser ignition. Nanoscale metalparticles, carbon nanotubes, high-nitrogen materials, and thin films ofnano-porous silicon are known to ignite with a broadband light source.Known methods of flash ignition have been successful with loose powdersor low-density materials. However, loose powders or low-densitymaterials are difficult to integrate into practical energetic systems.Specifically, it is challenging to adhere a loose powder to apropellant; and low-density composites consume more volume and may nothave robust mechanical integrity.

Laser ignition of solid propellants is a compelling alternative to knownelectrical ignition systems because laser ignition systems may enablemore precise spatial and temporal control over the ignition process.Laser ignition systems may also be implemented into practical energeticsystems more easily due to their higher energy output capabilitiescompared to broadband light sources. However, the sensitivity ofenergetic materials to energy flux from a laser can lead to a variety ofproblems during ignition. For example, double-base nitrocellulosepropellants require a critical and narrow window of flux intensity, an“ignition corridor,” for self-sustained ignition. Below the criticalenergy level, the propellant will not ignite. Above, the reaction can beoverdriven, where the surface burns faster than it would in normaldeflagration experiments. Thus, when the input laser energy is removedquickly, the burning propellant surface quickly drops in burning rateand, in some cases, is unable to recover to a steady burning rateresulting in extinguishment. Similar regimes of ignition were found inaluminized ammonium perchlorate (AP) composite propellant. Weber et al.,Radiative ignition and extinction dynamics of energetic solids, J.Thermophys. Heat Transf. 19 (2005) 257-265, studied this effectcomputationally and compared the results tocyclotetramethylene-tetranitramine (HMX), and concluded that sustainedlaser ignition of propellants is achieved by satisfying two conditions:(1) the surface must reach a critical activation temperature forcondensed phase surface decomposition, and (2) the thermal profile, orpre-heat zone of the propellant, must be able to recover tonon-radiation augmented conditions when the laser input is removed. Thethermal profile of the propellant is largely tied to condensed-phaseheat transfer and chemical processes, so the time of laser radiation, aswell as how fast the laser radiation is removed, can both have aneffect. This was confirmed experimentally by Ali et al., High-irradiancelaser ignition of explosives, Combust. Sci. Technol. 175 (2003)1551-1571, who applied the dual ignition criteria model (DICM) topredict the ignition energy needed for HMX to within ten percent ofmeasured values, which requires both a critical temperature and energydeposition approximately equal to the thermal energy in preheat zone ina steadily burning propellant. Other important parameters for ignitiondelay include pressure, the absorptivity of the propellant, and materialformulation.

Both ignition delay control and self-sustained ignition are crucial forpractical propellant systems. A possible solution for consistent laserignition is the use of an absorptive coating or by adding photosensitiveparticles to the propellant to reach critical surface temperatures morerapidly. Carbon black, carbon nanotubes, high nitrogen materials, andnanometal particles have been identified for their sensitivity to laserignition, although with varying degrees of success. Carbon black hasbeen used extensively to increase the absorptivity of propellants at thesurface. When used as a coating or additive, carbon black has improvedignition in a range of wavelengths from 500-1064 nm. Additionally,carbon black has been used to reduce ignition thresholds and delays insecondary explosives. However, the use of carbon black is limited inenergetic materials due to its typical small particle size, which canlead to increased porosity and agglomeration. Furthermore, carbon blackis a relatively inert material, resulting in decreased energeticperformance. Switching to energetic sensitizers raises issues ofabsorptivity, as energetic materials are rarely absorptive towavelengths below infrared. Many energetic materials may be ignitedusing high-powered carbon dioxide lasers operating with wavelengths inthe range of 10.6 μm, but these wavelengths are often not practical forenergetic applications. Using lower energy lasers operating atwavelengths from ranges of 200-1200 nm generally requires loose powderor porous materials, similar to flash ignition, and these materials areunable to achieve sustained ignition when the laser radiation isremoved. Sustained ignition may be understood as when a stable flame ismaintained for at least five seconds. Sustained ignition is oftendifficult to achieve in propellants due to a lack of opticalabsorptivity at convenient wavelengths.

There is a continuing need for a low-energy ignition system that iscapable of achieving a consistent and sustained ignition of apropellant. Desirably, the absorptivity of the low-energy ignitionsystem may enable the use of a laser ignition source.

SUMMARY

In concordance with the instant disclosure, an igniter system configuredto be ignited by an optical energy source that is capable of achieving aconsistent and sustained ignition of a propellant, has been surprisinglydiscovered.

In one embodiment, an igniter system configured to be ignited by anoptical energy source includes an igniter composite and a propellant.The igniter composite may include a reactive component and afluoropolymer. The propellant may be coupled to the igniter composite.In a specific example, the optical energy source may be a laser sourceand/or an incoherent (e.g., flash-lamp) source. The igniter compositemay be configured to achieve a sustained ignition from a wavelengthemitted from the optical energy source. The wavelength may be betweenaround 250 nanometers to around 1100 nanometers. In certaincircumstances, the igniter composite may be formed as a layer disposedon the propellant. In a specific example, the igniter composite may beformed as a first layer disposed on a second layer. In a more specificexample, the first layer may include smaller particles of the ignitercomposite in comparison to the particles of the igniter composite withinthe second layer. In an alternative example, the igniter composite maybe formed as composite pellets which may then be disposed within and/orsubstantially throughout the propellant.

In another embodiment, the present technology includes methods of usingthe igniter system. For instance, a first method of manufacturing theigniter system may include providing an igniter composite including areactive component and a fluoropolymer. The igniter composite may beconfigured to achieve a sustained ignition from a wavelength emittedfrom the optical energy source. The wavelength may be between around 250nanometers to around 1100 nanometers. The first method may also includea step of providing a propellant. The igniter composite may then becoupled to the propellant.

In certain circumstances, the igniter system may be used according to asecond method. The second method may include providing the ignitersystem including a propellant and an igniter composite. The ignitercomposite further including a reactive component and a fluoropolymer.The propellant may be coupled to the igniter composite. The ignitercomposite may be configured to achieve a sustained ignition from awavelength emitted from the optical energy source. The wavelength may bebetween around 250 nanometers to around 1100 nanometers. Next, thesecond method may include applying a form of optical energy from theoptical energy source to the igniter system. Then, the igniter compositemay be ignited. The second method may include another step of ignitingthe propellant.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic diagram of an igniter system, further depicting anigniter composite as a single first composite layer having a firstreactive component and a first fluoropolymer, according to oneembodiment of the present disclosure;

FIG. 2 is a schematic diagram of the igniter system, further depictingthe first composite layer having the first reactive component and thefirst fluoropolymer, and a second composite layer having a secondreactive component and a second fluoropolymer, wherein the firstreactive component is different from the second reactive component, andthe first fluoropolymer is different from the second fluoropolymer,according to one embodiment of the present disclosure;

FIG. 3 is a schematic diagram of the igniter system, further depictingthe first composite layer having the first reactive component and thefirst fluoropolymer, and the second composite layer having the secondreactive component and the second fluoropolymer, wherein the firstreactive component is the same as the second reactive component, and thefirst fluoropolymer is the same as the second fluoropolymer, accordingto one embodiment of the present disclosure;

FIG. 4 is a schematic diagram of the igniter system, further depictingthe igniter composite as a first composite layer and a second compositelayer disposed on a propellant, according to one embodiment of thepresent disclosure;

FIG. 5 is a schematic diagram of the igniter system, further depictingthe igniter composite as a particle disposed in the propellant,according to one embodiment of the present disclosure;

FIG. 6 is a schematic diagram of the flash-activated setup for ignitingand analyzing the igniter system sample, according to one embodiment ofthe present disclosure;

FIG. 7 is a schematic diagram of the laser-activated setup for ignitingand analyzing the igniter system sample, according to one embodiment ofthe present disclosure;

FIG. 8 is a microscopic image of the single first composite layer havinga printed nano-scale aluminum (nAl) composition, according to oneembodiment of the present disclosure;

FIG. 9 is a microscopic image of the first composite layer having aprinted nAl composition and a second composite layer having a printedmicron-scale aluminum (μAl) composition, according to one embodiment ofthe present disclosure;

FIG. 10 is an interval plot illustrating a minimum ignition energy (MIE)of igniter composites that were printed and tape-casted at varyingconcentrations of nAl and μAl, further depicting a dashed line toindicate the minimum nAl content necessary, according to one embodimentof the present disclosure;

FIG. 11 is a line graph illustrating profilometer measurements obtainedfrom the igniter composites that were printed and tape-casted withvarying surface roughness, according to one embodiment of the presentdisclosure;

FIG. 12 is the line graph, as shown in FIG. 11 , illustrating theindividual profilometer measurements, further depicting the largeramplitude variations in surface height correspond to lower MIE and thatno significant difference in MIE was observed between the rough sidesand the smooth sides of the nAl tape-casted films, according to oneembodiment of the present disclosure;

FIG. 13 is a line graph illustrating an intensity trace of a flashignited propellant sample exhibiting extinguishment, according to oneembodiment of the present disclosure;

FIG. 14 is a line graph illustrating an intensity trace of a flashignited propellant sample exhibiting delayed ignition, according to oneembodiment of the present disclosure;

FIG. 15 is a time progression of images depicting a printed nAl igniterexhibiting delayed ignition, according to one embodiment of the presentdisclosure;

FIG. 16 is a line graph illustrating an intensity trace of a flashignited propellant sample exhibiting continuous ignition, according toone embodiment of the present disclosure;

FIG. 17 is a time progression of images depicting a printed ignitercomposite exhibiting continuous ignition, according to one embodiment ofthe present disclosure;

FIG. 18 is an interval plot illustrating a total ignition delay offlash-ignited printed igniter composites, further depicting a percentageof continuous ignition occurrences for each sample, according to oneembodiment of the present disclosure;

FIG. 19 is an interval plot illustrating a laser ignition delay ofprinted igniter composites, further depicting a percentage of continuousignition occurrences for each sample, according to one embodiment of thepresent disclosure;

FIG. 20 is a line graph illustrating an intensity trace of a laserignited propellant sample exhibiting continuous ignition, according toone embodiment of the present disclosure;

FIG. 21 is a line graph illustrating an intensity trace of a laserignited propellant sample exhibiting delayed ignition, according to oneembodiment of the present disclosure;

FIG. 22 is a line graph illustrating an intensity trace of a laserignited propellant sample exhibiting extinguishment, according to oneembodiment of the present disclosure;

FIG. 23 is a flowchart of a first method for manufacturing the ignitersystem, further depicting a decision if the igniter composite will bedisposed on the propellant or if the igniter composite will be disposedwithin the igniter composite, according to one embodiment of the presentdisclosure; and

FIG. 24 is a flowchart of a second method for using the igniter system,according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

The following description of the technology is merely exemplary innature of the subject matter, manufacture, and use of one or moreinventions, and is not intended to limit the scope, application, or usesof any specific invention claimed in this application or in such otherapplications as may be filed claiming priority to this application, orpatents issuing therefrom. Regarding methods disclosed, the order of thesteps presented is exemplary in nature unless otherwise disclosed, andthus, the order of the steps can be different in various embodiments,including where certain steps can be simultaneously performed.

I. Definitions

Unless otherwise defined, all technical terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich the present disclosure pertains.

As used herein, the terms “a” and “an” indicate “at least one” of theitem is present; a plurality of such items may be present, whenpossible. Except where otherwise expressly indicated, all numericalquantities in this description are to be understood as modified by theword “about” and all geometric and spatial descriptors are to beunderstood as modified by the word “substantially” in describing thebroadest scope of the technology. “About” when applied to numericalvalues indicates that the calculation or the measurement allows someslight imprecision in the value (with some approach to exactness in thevalue; approximately or reasonably close to the value; nearly). If, forsome reason, the imprecision provided by “about” and/or “substantially”is not otherwise understood in the art with this ordinary meaning, then“about” and/or “substantially” as used herein indicates at leastvariations that may arise from ordinary methods of measuring or usingsuch parameters.

Although the open-ended term “comprising,” as a synonym ofnon-restrictive terms such as including, containing, or having, is usedherein to describe and claim embodiments of the present technology,embodiments may alternatively be described using more limiting termssuch as “consisting of” or “consisting essentially of” Thus, for anygiven embodiment reciting materials, components, or process steps, thepresent technology also specifically includes embodiments consisting of,or consisting essentially of, such materials, components, or processsteps excluding additional materials, components or processes (forconsisting of) and excluding additional materials, components orprocesses affecting the significant properties of the embodiment (forconsisting essentially of), even though such additional materials,components or processes are not explicitly recited in this application.For example, recitation of a process reciting elements A, B and Cspecifically envisions embodiments consisting of, and consistingessentially of, A, B and C, excluding an element D that may be recitedin the art, even though element D is not explicitly described as beingexcluded herein.

As referred to herein, disclosures of ranges are, unless specifiedotherwise, inclusive of endpoints and include all distinct values andfurther divided ranges within the entire range. Thus, for example, arange of “from A to B” or “from about A to about B” is inclusive of Aand of B. Disclosure of values and ranges of values for specificparameters (such as amounts, weight percentages, etc.) are not exclusiveof other values and ranges of values useful herein. It is envisionedthat two or more specific exemplified values for a given parameter maydefine endpoints for a range of values that may be claimed for theparameter. For example, if Parameter X is exemplified herein to havevalue A and also exemplified to have value Z, it is envisioned thatParameter X may have a range of values from about A to about Z.Similarly, it is envisioned that disclosure of two or more ranges ofvalues for a parameter (whether such ranges are nested, overlapping, ordistinct) subsume all possible combination of ranges for the value thatmight be claimed using endpoints of the disclosed ranges. For example,if Parameter X is exemplified herein to have values in the range of1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may haveother ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10, 2-8, 2-3,3-10, 3-9, and so on.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected, or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to” or “directly coupled to” another element orlayer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer, or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer, or section discussed below could be termed a second element,component, region, layer, or section without departing from theteachings of the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below”, or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

As used herein, the term “sustained ignition” refers to when a desiredmaterial for ignition continues to burn after the input energy isremoved. Sustained ignition may also be understood as where the reactionwas not interrupted and/or did not necessitate a secondary initiation.

As used herein, the term “ignition delay” refers to the time from firstlight on the igniter to the observation of steady flame propagationacross the propellant.

As used herein, the term “wt. %” refers to weight percent, which is theamount of a specified substance in a unit amount of another substance.

As used herein, the term “μAl” refers to micron-scale aluminum.

As used herein, the term “nAl” refers to nano-scale aluminum.

As used herein, the term “Al/PVDF igniter material” refers to aparticular aluminum/polyvinylidene fluoride composition used in aspecific embodiment of an igniter composite.

As used herein, the term “laser” refers to a device that generates anintense beam of coherent monochromatic light (or other electromagneticradiation) by stimulated emission of photons from excited atoms ormolecules.

As used herein, the term “incoherent” refers to a type of source thatemits light with frequent and random changes of phase between thephotons.

In the present disclosure the terms “about” and “around” may allow for adegree of variability in a value or range, for example, within 10%,within 5%, or within 1% of a stated value or of a stated limit of arange.

In the present disclosure the term “substantially” can allow for adegree of variability in a value or range, for example, within 90%,within 95%, or within 99% of a stated value or of a stated limit of arange.

II. Description

As shown in FIGS. 1-7 , an igniter system 100 configured to be ignitedby an optical energy source 102 includes an igniter composite 104, 106and a propellant 108. The igniter composite 104, 106 may include areactive component 110, 112 and a fluoropolymer 114, 116. In certaincircumstances, the propellant 108 may be constructed from a propellantslurry and thereafter formed into a solid form of the propellant 108.The propellant 108 may be coupled to the igniter composite 104, 106. Theigniter composite 104, 106 may be configured to achieve a sustainedignition from a wavelength emitted from the optical energy source 102.The wavelength may be between around 250 nanometers to around 1100nanometers. In a specific example, the optical energy source 102 may beone of a laser source and/or an incoherent (e.g., flash-lamp) source. Inanother specific example, the igniter composite 104, 106 may beconfigured to achieve the sustained ignition with less than five joulesper square centimeter of energy provided by the optical energy source102. In an even more specific example, the igniter composite 104, 106may be configured to achieve the sustained ignition with less than seventenths of a joule per square centimeter of energy provided by theoptical energy source 102. The igniter system 100 may also be configuredto achieve the sustained ignition with a wavelength from the opticalenergy source of around 250 nanometers to around 1100 nanometers. Morespecifically, the igniters system 100 may achieve the sustained ignitionwith a laser wavelength of around 1064 nanometers and/or around 532nanometers. Advantageously, the igniter composite 104, 106 of theigniter system 100 may be configured to reduce the overall weight of theigniter system 100 and militate against electrical risks of traditionalinitiators, thereby resulting in more efficient and reliable solidrocket motor ignition systems. Desirably, the igniter system 100 mayprovide enhanced spatial and temporal control over the ignition process.

The igniter composite 104, 106 may include various components and havecertain functions. For instance, the reactive component 110, 112 of theigniter composite 104, 106 may include photoreactive materials and/orenergetic materials as a source of ignition for the propellant 108. Thereactive component 110, 112 may include at least one of a reactive metaland a metal oxide. More specifically, the reactive component 110, 112may include lithium, boron, sodium, magnesium, aluminum, silicon,calcium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel,copper, zinc, gallium, yttrium, zirconium, niobium, molybdenum,ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony,iodine, cesium, barium, hafnium, tantalum, tungsten, platinum, gold,mercury, lead, bismuth, any oxides of the metals, and/or any combinationthereof. In an even more specific example, the reactive component may bealuminum. Advantageously, the present disclosure demonstrates thetunability of the ignition delay and propagation properties ofoptically-sensitive, nearly full density reactivealuminum/polyvinylidene fluoride (Al/PVDF) films and additivelymanufactured igniter composites 104. A skilled artisan may select othersuitable reactive components, within the scope of the presentdisclosure.

The fluoropolymer 114, 116 may include various components and havecertain functions. For instance, the fluoropolymer 114, 116 may includePVF (polyvinylfluoride), PVDF (polyvinylidene fluoride), PTFE(polytetrafluoroethylene), PCTFE (polychlorotrifluoroethylene), PFA(perfluoroalkoxy polymer), [P(VDF-TrFE)] (poly(vinylidenefluoride-trifluoroethylene)), [P(VDF-TrFE-CFE)] (poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene)), THV (a polymer oftetrafluoroethylene, hexafluoropropylene and vinylidene fluoride), FEP(fluorinated ethylene-propylene), ETFE(polyethylenetetrafluoroethylene), HTE (a polymer ofhexafluoropropylene, tetrafluoroethylene and ethylene), ECTFE(polyethylenechlorotrifluoroethylene), FFPM/FFKM (PerfluorinatedElastomer), FPM/FKM (Fluorocarbon [Chlorotrifluoroethylenevinylidenefluoride]), FEPM (Fluoroelastomer [Tetrafluoroethylene-Propylene]), PFPE(Perfluoropolyether), PFSA (Perfluorosulfonic acid), and any combinationthereof. In a specific example, the fluoropolymer 114, 116 may be PVDF.One skilled in the art may select other suitable fluoropolymers, withinthe scope of the present disclosure.

In certain circumstances, the igniter composite 104, 106 may be formedas a composite layer 118, 120 disposed on the propellant 108, as shownin FIG. 1 . The igniter composite 104, 106 formed as a composite layer118, 120 may include a nearly full-density photosensitive reactivecomponent 110, 112. In a particular embodiment, the igniter composite104, 106 may include nano-aluminum (nAl) at ideal stoichiometry inpolyvinylidene fluoride (PVDF). The composition of nAl/PVDF provides afull-density igniter composite 104, 106 that retains its flash ignitioncapabilities, due to the PVDF binder isolating the nAl particles and thepre-ignition reactions between nAl and the fluoropolymer 114, 116. Asshown in FIG. 1 , a single first composite layer 118 of nAl/PVDF ignitercomposite 104, 106 was found to flash ignite but frequently yieldeddelayed transitions in steady propagation from the igniter composite104, 106 to the propellant 106. To improve the continuity and steadinessof the transition, fuel particle size, igniter thickness, and acombination of layers 118, 120 of nAl and μAl in PVDF were investigated.In a specific example, the reactive component 110, 112 may include afirst reactive component 110 and a second reactive component 112. Thefirst reactive component 110 and the second reactive component 112 maybe the same material, as shown in FIG. 3 , or they may be differentmaterials, as shown in FIG. 2 . The fluoropolymer 114, 116 may include afirst fluoropolymer 114 and a second fluoropolymer 116. The firstfluoropolymer 114 and the second fluoropolymer 116 may be the samematerial, as shown in FIG. 3 , or they may be different materials, asshown in FIG. 2 . The igniter composite 104, 106 may include a firstcomposite layer 118 and a second composite layer 120. The firstcomposite layer 118 may have a first composite layer particle 122including the first reactive component 110 and the first fluoropolymer114. The second composite layer 120 may include a second composite layerparticle 124 including the second reactive component 112 and the secondfluoropolymer 116. As shown in FIGS. 2-4 , the second composite layer120 may be disposed between the first composite layer 118 and thepropellant 108. In a more specific example, the first composite particle122 of the first composite layer 118 may be smaller particles incomparison to the second composite particles 124 of the second compositelayer 120. For instance, the first composite layer particle 112 may be asubstantially nano-sized particle. The second composite layer particle124 may be a substantially micro-sized particle. The total thickness ofthe first composite layer 118 and/or the second composite layer 120 maybe between at least five-hundredths of a millimeter to aroundfive-tenths of a millimeter. In an even more specific example, the firstcomposite layer 118 of nAl may be used to flash ignite the ignitercomposite 104, 106. The ignition of the first composite layer 118 maythen propagate to the second composite layer 120 of μAl without delay.In other words, the first composite layer 118 may be configured as anoptically sensitive layer that has a higher reaction rate and isconfigured to be ignited from an optical energy source. The secondcomposite layer 120 may be configured to have a lower reaction rate andmay be less optically sensitive in comparison to the first compositelayer 118, which enhances the second composite layer 120 ability toignite the propellant 108. It should be appreciated the first compositelayer 118 and the second composite layer 120 may each further include aplurality layers. For instance, where the igniter composite 104, 106 iscast onto strands of ammonium perchlorate composite propellant,continuous ignition may be achieved with a single first composite layer118 of nAl printed atop a plurality of second composite layers 120 ofμAl for a flash-activated propellant and/or a laser-activatedpropellant. In a specific example, the plurality of layers within eachof the first composite layer 118 and/or the second composite layer 120may be between at least five-hundredths of a millimeter to aroundfive-tenths of a millimeter. Without being bound to any particulartheory, the nAl/PVDF first composite layer 118 is believed to enablegood flash and/or laser ignition sensitivity. Further, the μAl/PVDFsecond composite layer 120 is believed to produce a more sustained heattransfer to thereby produce a more reliable ignition process. Theigniter composite 104, 106 may be coupled to the propellant 108 throughvarious ways. For instance, the igniter composite 104, 106 may beinitially constructed via tape casting as a film. The igniter composite104, 106 may also be formed through additive manufacturing/3D printing.In a specific example, the igniter composite 104, 106 may be initiallyconstructed separately from the propellant 108 via tape casting and/or3D printing, and thereafter coupled to the propellant 108 during apropellant curing process. The propellant curing process may includesolidifying a slurry with a liquid binder to form the propellant 108.Advantageously, 3D printing the igniter composite 104, 106 may providemore precise control over the geometry and thickness of the ignitercomposite 104, 106. In an alternative embodiment, the igniter composite104 may be disposed within the slurry mixture during the propellantcuring process so that igniter composite 104 will then be disposedwithin the propellant 108. Desirably, providing the igniter composite104 within the propellant 108 may enhance the efficiency ofmanufacturing the igniter system 100.

With continued reference to the alternative embodiment, as shown in FIG.5 , an optically sensitive igniter composite 104, 106 material with ahigher reaction rate may be used directly as an additive in thepropellant 108 to enable optical ignition. The igniter composite 104,106 may be formed as the first composite particle 122 which may then bedisposed within the propellant 108. In a specific example, the firstcomposite particle 122 may be shaped as a composite pellet. Thecomposite pellet may also be provided as a nano-sized first compositeparticle 122 disposed in the propellant 108. The nano-sized firstcomposite particle 122 may include thirty weight percent (wt. %) tofifty wt. % of the reactive component 110, 112. In certaincircumstances, the first composite particle 122 may include nAl/PVDF asthe optically sensitive reactive component 110, 112. In a more specificexample, the first composite particle 122 may comprise around forty wt.% nAl. The first composite particle 122 may be less than one millimeterin diameter. In a more specific example, ammonium perchlorate(AP)/hydroxyl-terminated polybutadiene (HTPB) composite propellant 108is optically sensitized by adding the energetic nAl/PVDF first compositeparticles 122, which may lead to enhanced sustained ignition withrelatively low radiative energy levels (<5 J/cm2). With continuedreference to the more specific example, the nAl/PVDF propellant 108 wascompared to a neat AP/HTPB propellant having a carbon black additive andalso compared to a propellant with a nano-aluminum (nAl) additivewithout PVDF. Only the nAl/PVDF propellant 108 exhibited sustainedignition. The nAl/PVDF first composite particles 122 were characterizedusing optical imaging and ranged from 600 to 1000 μm. The ignitiondelays of the propellant 108 with nAl/PVDF additives ranged from 1.8 msto 4.5 ms, depending on the energy density of the laser 102. Theseoptically sensitive additives offer improved ignition for propellantsand exhibit sustained burning over a wide range of laser energies. Theuse of nAl/PVDF as the optically sensitive igniter composite 104, 106within a first composite particle 122 may enable the optical ignition ofthe propellant 108 without the use of any layered igniter, therebyenabling the propellant 108 to be ignited directly from an opticalenergy source 102. Advantageously, the first composite particle 122 doesnot require the use of a second layer and/or a material with a lowerreaction rate which simplifies the ignition process of propellants 108even further.

In another embodiment, the present technology includes methods of usingthe igniter system 100. For instance, as shown in FIG. 23 , a firstmethod 200 of manufacturing the igniter system 100 may include providingan igniter composite 104, 106 including a reactive component 110, 112and a fluoropolymer 114, 116. The igniter composite 104, 106 may beconfigured to achieve a sustained ignition a wavelength between around250 nanometers to around 1100 nanometers from an optical energy source102. The first method 200 may also include a step 204 of providing apropellant 108 as a propellant slurry. The igniter composite 104, 106may then be coupled to the propellant slurry. In certain circumstances,a decision may be made if the igniter composite will be disposed withinthe propellant or if the igniter composite will be disposed on thepropellant. In a specific example, the igniter composite 104, 106 may bemanufactured utilizing additive manufacturing and/or tape-casting andmay thereafter be disposed on the propellant slurry during a propellantcuring process. Then, the propellant slurry may be solidified with theigniter composite 104, 106 disposed thereon. In an alternative example,the igniter composite 104, 106 may be coupled to the propellant 108 bydisposing the igniter composite 104, 106 within the propellant slurry.For instance, the igniter composite 104, 106 may be provided as a firstcomposite particle 122 and the first composite particle 122 may bedisposed within the propellant slurry. In a more specific example, aplurality of first composite particles 122 may be disposed randomlyand/or substantially evenly throughout the propellant slurry. The method200 may then further include a step 212 of solidifying the firstcomposite particle(s) 122 of the igniter composite 104, 106 within thepropellant slurry.

In certain circumstances, the igniter system 100 may be used accordingto a second method 300, as shown in FIG. 30 . The second method 300 mayinclude providing the igniter system 100 including a propellant 108 andan igniter composite 104, 106 including a reactive component 110, 112and a fluoropolymer 114, 116. The propellant 108 may be coupled to theigniter composite 104, 106. The igniter composite 104, 106 may beconfigured to achieve a sustained ignition from a wavelength emittedfrom the optical energy source 102. The wavelength may be between around250 nanometers to around 1100 nanometers. Next, the second method 300may include applying a form of radiative energy from the optical energysource 102 to igniter system 100. Then, the igniter composite 104, 106may be ignited. The second method 300 may include another step 308 ofigniting the propellant 108.

III. Example

Provided as non-limiting examples, FIGS. 8-22 illustrate variousspecific examples of the igniter system 100. For instance, thin films ofigniter composite 104, 106 having around twenty percent weight Al/PVDFwere prepared with nAl and μAl fuel. Nine formulations of the ignitercomposite 104, 106 films were made by varying the active nAl (80 nmnominal diameter, 70% active content, Novacentrix) content in the fuelby 12.5 wt. % with the remaining fuel consisting of μAl (3 μm, ValimetH-3). Powdered PVDF (Kynar 711, Arkema) was dissolved indimethylformamide (DMF) at a solvent ratio of 6:1 mL per gram of PVDF.Aluminum was added to DMF then ultrasonically mixed using a BRANSONDIGITAL SONIFIER® for three minutes before adding PVDF and followingwith a second mixing cycle. The solution was cast into thin films usingan MIT Corporation MSK-AFA-HC100™ tape caster with a heated bed held at125° C. to ensure full-density igniter composite 104, 106 films. Theigniter composite 104, 106 films were dried on the heated bed forfifteen minutes before removing and drying for several hours at ambientconditions. The microstructure of the tape cast films was observed usingscanning electron microscopy (SEM), more particularly an FEI NovaNanoSEM® at an operating energy of 5 kV. The samples were coated with a20 nm layer of platinum (Pt) using a CRESSINGTON™ sputter coater.

Seven structures of igniter systems 100 were fabricated for ignitiondelay testing on propellants 108: two tape-cast and five printedformulations. The igniter composite 104, 106 formulations had an activecontent of 20 wt. % aluminum in PVDF which was determined to be close tostoichiometric. The tape-cast films consisted of pure nAl and a mixtureof nAl and μAl fuel in which 75 wt. % of the fuel content was nAl and 25wt. % μAl (0.75 nAl film), resulting in thicknesses between 20-30 μm.Printed igniter formulations included a single first layer 118 nAl (1nAl), a five-stacked first layer 118 nAl (5 nAl), a single first layer118 nAl on top of a single second layer 120 of μAl (1 nAl×1 μAl), asingle first layer 118 nAl on top of three second layers 120 of μAl (1nAl×3 μAl) and a single second layer 120 nAl and μAl mixture (0.75 nAl).

Printable filament material was printed into 1 cm×1 cm igniters throughfused-filament fabrication (FFF) with a print speed of 10 mm/s and layerheight of 0.125 μm. The filament was passed through an extruder headheated to 240° C. and deposited onto a heated build plate maintained at70° C. with a BUILDTAK® 3D Printing Surface supplemented by a layer ofglue, such as ELYMER'S™ All Purpose Glue.

The igniter composite 104, 106 was cast onto the propellant 108comprising of 85 wt. % AP with a 4:1 coarse (60-130 μm, Firefox) to fine(20 μm, ATK) ratio and 15 wt. % binder. The binder consisted of 76.33wt. % hydroxyl-terminated polybutadiene (R45-M HTPB, Rocket MotorComponents), 15.05% isodecyl pelargonate plasticizer (Rocket MotorComponents), and 8.62% isophorone diisocyanate curing agent (Firefox).Batches of 35 g were prepared with an initial hand mixing followed bytwo cycles on a LABRAM® RESODYN® resonant mixer for three minutes at 80g under vacuum. The propellant 108 was cast into apolytetrafluoroethylene mold with dimensions of 2.54 cm×7.62 cm×0.64 cmwith one side exposed. The igniter composite 104, 106 was disposed ontop of the propellant 108 with the exposed top nAl first composite layer118 then covered. The mold was compressed until the propellant 108 nolonger pushed through overflow holes. No intermediate adhesive wasneeded to hold the igniter composite 104, 106 in place as the curedpropellant 108 secured the igniter composite 104, 106 in place directly.In this specific example, a minimum of ten propellant 108 samples werefabricated for each igniter composite 104, 106 formulation.

Flash testing of the nAl and μAl mixture films was conducted using aWhite Lightning X3200™ broadband xenon flash lamp commercially availablethrough Paul C. Buff Inc. The films were cut into 1 cm×1 cm squares andplaced in front of the flash bulb at a set distance. Due to thescattering of the broadband light, the deposited energy was controlledby distance, up to a maximum of 2 cm. The flash lamp was then triggered,and ignition of the films was selectively observed. The placement ofeach sample was determined using Neyer's Sensitivity Test. A minimum of30 samples were tested to determine the minimum ignition energy (MIE)resulting in a 50% probability of ignition with a two-sided confidenceinterval of 95%. These tests were then repeated for nAl printed ignitercomposites 104, 106 of the same dimensions.

As shown in FIGS. 6-7 , a similar test setup was utilized for flash andlaser ignition of the propellants 108. As shown in FIG. 6 , thepropellants 108 were placed below the flash lamp 102 with the ignitercomposite 104, 106 facing the xenon flash bulb 102 or the laser 102. Athin glass slide was placed between the propellant 108 and the flashbulb 102 to protect the flash bulb 102 from the high temperatures of thepropellant flame. The flash lamp 102 was then triggered and theresulting ignition was captured at 2500 fps with a PHANTOM™ v2012high-speed camera. The incident flash energy was roughly 7 J/cm2.Likewise, tests were then repeated with a laser system 102, as shown inFIG. 7 , to observe ignition from a highly focused heating sourcewithout the need of a protective glass slide. The samples were ignitedwith a Nd:YAG laser 102 operating at a wavelength of 1064 nm and energyof 7 J/cm2 fired in a pulse burst mode with a 5 ms burst, 10 ns pulses,and a repetition rate of 100 kHz. These settings roughly match thetemporal profile of the flash bulb.

Surface roughness measurements were obtained on films and 3D printedmaterials using an ALPHA-STEP® D-600 profilometer. The profilometer wasable to detect changes in surface height on the scale of nanometers andproduce profile traces of the materials. These profile traces were usedto capture general roughness and surface features to compare the filmsto the printed materials that would be exposed to the optical energyinput.

The igniter composite 104, 106 needs to consistently initiate thepropagation of the energetic material in the propellant 108 to achievesustained ignition. Dimming, quenching, and unsteady transitions areundesirable as they lead to increased variability in performance. Thesecond primary factor is the delay between initiation and ignition ofthe energetic material. To characterize the Al/PVDF film and printedigniter composites 104, 106, the minimum ignition energy (MIE), ignitiondelay, and consistency of transition from the igniter composite 104, 106to an AP composite propellant 108 were quantified.

Printed igniter composites 104, 106 with layers of nAl and μAl wereexamined with SEM to investigate the surface and internal structure ofthe samples. As shown in FIG. 8 , the surface of the printed nAl ignitercomposite 104, 106 had a higher roughness than the tape-cast films ofthe same formulation. To examine a dual-layer cross-section, the ignitercomposites 104, 106 were cut with a razor blade. As shown in FIG. 9 ,the nAl and μAl layers were identifiable by the microstructure. In thelayer of μAl, Al particles can be seen displaced from their originalpositions due to the blade cut, and the tracks left during thedisplacement were not indicative of porosity before the cut.

As shown in FIG. 10 , the effect of Al size on flash ignition can beseen. There are two key thresholds for flash ignition: the minimum massfraction of nAl needed to flash ignite and the asymptotic decline of MIEas nAl increased. This is believed to be caused by the interaction oflight energy in the film being scattering-dominated with μAl butabsorption-dominated with nAl, and a critical amount of energy must beabsorbed for the igniter composite 104, 106 to ignite. Below a massfraction of 0.375 nAl, the flash setup was unable to ignite any films,as too little energy was absorbed by the nAl. Utilizing a flash bulbwith a higher energy level or different flash duration are contemplatedto provide the energy needed for ignition below this setup-limitedthreshold. At a mass fraction 0.625 nAl, the MIE dropped to ˜5 J/cm2 andremained essentially constant as the nAl mass fraction increased. Thevariability also decreased for 0.625 nAl and above. That is, althoughignition was observed reliably at a nAl mass fraction of 0.375 and 0.50,these samples had higher MIE and variability. However, once the nAl massfraction reached 0.625, the highly absorptive nAl particles dominatedthe interaction with the light energy. While there was still some μAl,the light scattering did not hinder the absorption and the MIE remainedconstant. Additionally, the MIE for the 1 nAl printed igniter wassignificantly lower than films of the same composition.

The surface roughness of the igniter composite 104, 106 was discoveredto affect the sensitivity to optical ignition. The process oftape-casting nAl igniter composite 104, 106 samples produced two surfacefinishes: the side of the film exposed to air produced a relativelyrough surface, while the side of the film touching the glass plateproduced a smooth surface. Additionally, printed Al/PVDF ignitercomposite 104, 106 samples had a rougher surface with higher-amplitudesurface variation than the films. A printed Al/PVDF igniter composite104, 106 sample was carefully sanded with 240 grit sandpaper to createsmoother features and all four of these surfaces were measured using aprofilometer. Higher amplitude surface height variations, as shown inFIGS. 11-12 , corresponded with the lowered MIE shown in FIG. 10 .Likewise, this can also be seen with the rough printed samples. Therough printed samples had a visibly rough surface and thehighest-amplitude peaks in the surface height due to the prescribedprint lines and nozzle limitations. The high-amplitude variation had atwo-fold effect that led to increased sensitivity. The high peakscreated a blackbody effect as light reflected from the sample had ahigher chance of colliding with another peak in the surface, globallyincreasing the amount of energy absorbed by the surface and lowering theMIE. Secondly, the high peaks formed corners in which the heat from theabsorbing sides may not be conducted away as fast into the bulk of thesample, similar to a propellant 108 corner/edge burning effect, forminghotspots that led to ignition requiring less input energy than a flattersurface. To test whether the amplitude of the surface variations was theprimary driver decreasing the MIE, the printed igniter composite 104,106 samples were sanded with 180 grit sandpaper to eliminate the largepeaks. Sanded printed igniter composite 104, 106 samples exhibitedsurface height variations and MIE between that of the rough printed andthe rough films. The rough and smooth sides of the films showed nosignificant differences in MIE despite the overall increase in surfaceroughness in the rough films, reinforcing the observation that thehigher-amplitude variation of a surface can lead to an increasedabsorption of reflected light and/or hotspot formation.

The Al/PVDF igniter composite 104, 106 commonly exhibited three modes oftransition when igniting AP composite propellant 108: 1) continuous(prompt) ignition, 2) delayed ignition, and 3) extinguishment (failureto ignite). A continuous ignition is characterized by a steadypropagation of the igniter material to a steady self-propagatingreaction of the propellant 108 pellet. Specifically, a minimal delay isobserved between first light of the igniter composite 104, 106 and fullignition of the propellant 108. A delayed ignition is typified by adiminished reaction between the igniter composite 104, 106 beforerecovery to steady propellant propagation, as indicated by a drop inlight emission before recovering across the propellant 108 pellet.Extinguishment is characterized as the consumption of the ignitercomposite 104, 106 but unsuccessful ignition of the propellant 108pellet.

The aluminized igniter composite 104, 106 material burned brighter andhad a different flame structure than that of the AP propellant 108flames. This visual difference made the identification of the quality ofthe ignition between the igniter composite 104, 106 and propellant 108possible, both visually and through the quantification of lightintensity. The manner of transfer—continuous ignition, delayed ignition,or extinguishment—could also be clearly visualized in both the imageframes and with the intensity traces. When igniting a sample with abroadband flash lamp, the bulb was exposed, leading to a saturation ornear saturation of the field of view. This was manifested in an initialspike of saturation, as shown in FIG. 13 . In all of the ignition eventswith the flash lamp, the igniter composite 104, 106 began to react asthe flash intensity decreased, making quantification of the delaybetween triggering the flash and the first light on the igniter materialdifficult. Ignition of the igniter composite 104, 106 material was thusdefined as the time at which the intensity had fallen to 10% of thehighest value of the first peak (flash). Similarly, the first light ofthe propellant 108 ignition appeared as the igniter composite 104, 106material was consumed and was thus demarcated as the time at which theintensity had fallen to 10% of the highest value of the second peak(igniter peak). After the second peak (igniter peak), the intensitytrace would either continue at a steady value, which indicatedcontinuous or prompt ignition, decrease in value before rising to anasymptotic steady state, which indicated a delayed ignition, or fall tonear zero, which indicated extinguishment.

With continued reference to FIG. 13 , all of the propellants 108 withfilm igniters resulted in extinguishment and failed to ignite thepropellant 108, as the film was consumed too quickly for sufficientenergy to transfer into the propellant 108 for a continued propagatingreaction. As described by Ali et al., High-irradiance laser ignition ofexplosives, Combust. Sci. Technol. 175 (2003) 1551-1571, in order to seesustained ignition and satisfy the dual ignition criteria model (DICM),the material must reach a critical surface temperature and depositedenergy. While the nAl/PVDF film burned at a high enough temperature, thereaction was too fast in this configuration to satisfy the criticalenergy criteria in the AP composite propellant 108. The burning rate ofnAl/PVDF and the limited thickness of 20-30 μm in the films resulted ininsufficient time for the AP composite propellant 108 to reach thecritical energy threshold needed to ignite.

As shown in FIG. 14 , there was a delayed ignition between the ignitercomposite 104, 106 and the propellant 108 pellet. This occurred in manyof the igniter composites 104, 106 having only nAl layers. Withcontinued reference to FIG. 14 , a decrease of nearly 46% below thesteady-state intensity value occurred after the second peak, during thetransition from the igniter composite 104, 106 material to thepropellant 108, before rising to an asymptote indicating a steady-statereaction had been achieved across the propellant 108. As shown in FIG.15 , this corresponded with a visibly noticeable dimming in thecorresponding image frames.

To address the need for the reliable production of a continuous ignitionfrom the reaction of the nAl layer to steady flame propagation in thepropellant 108, a layer of μAl was added below the nAl. Without beingbound to any particular theory, it is believed formulations of μAl reactslower than nAl; however, it is also believed μAl is not as flashignitable compared to nAl. This slower burning layer μAl increases thetime of the reaction and reduces the burning rate mismatch between thenAl layer and AP composite propellant 108, therefore satisfying thecritical energy criteria to ignite the AP composite propellant 108. Afirst composite layer 118 of nAl enhances the initiation the reaction. Asingle first composite layer 118 of nAl exhibited continuous ignitioninto the second composite layer 120 of μAl beneath the first compositelayer 118. On average, igniter systems 100 with a combination of nAl andμAl layers 118, 120 were found to have an increased number of sampleswith continuous ignition of the propellant 108 pellet as compared to theigniter systems 100 with only nAl layers 118. In contrast to the delayedignition, continuous ignition did not exhibit a significant undershootas the intensity between the igniter composite 104, 106 material and thepropellant 108 approached the steady-state propellant intensity, asshown in FIGS. 16-17 .

Ultimately, the layered 1 nAl×1 μAl igniter system 100 performedsimilarly to the 5 nAl igniter system 100, as shown in FIG. 18 , with asimilar number of samples exhibiting delayed ignition between theigniter composite 104, 106 material and propellant 108. Increasing thenumber of μAl second composite layers 120 resulted in a longer totalignition delay than the 1 nAl×1 μAl igniter system 100 as more materialhad to be consumed before reaching the propellant 108 pellet. The 1nAl×3 μAl igniter system 100 samples had more consistent continuousignition, but also had the largest average and variance of totalignition delay of the formulations tested. Only one igniter system 100sample of 1 nAl×3 μAl exhibited a delayed ignition of the propellant108.

With continued reference to FIG. 18 , the printed igniter composites104, 106 consisting of 1 nAl and 5 nAl exhibited a delayed ignition ofthe propellant for 20% and 60% of the samples, respectively. Theincreased thickness of the 5 nAl resulted in a higher average totalignition delay than the 1 nAl (0.51 s and 0.31 s, respectively);however, the 5 nAl igniters led to more samples exhibiting continuousignition. A single layer igniter composite 104, 106 with a mixture ofnano- and micro-scale fuel, such as 75 wt. % nAl, was also investigatedto improve the consistency of the ignition but was found to exhibit asimilar percentage of continuous ignition samples as the 5 nAl with areduced total ignition delay time. This indicated that the speed ofreaction may not be solely responsible for determining the quality ofthe transition between the igniter composite 104, 106 material and thepropellant 108.

The total ignition delays in Table 1 are associated with the time fromfirst light on the igniter composite 104, 106 material to the recoveryof steady reaction across the propellant 108 pellet, as determined bythe intensity traces. Manual verification of the total ignition delaysled to an error associated with each test based on the visualidentification of the range of frames involving the first signs of APpropellant flames across the entire surface to the consumption of anylingering Al/PVDF. The associated error per test increased with theaddition of μAl layers due to the occasional lingering of burningtendrils of Al/PVDF on the surface of the propellant 108.

TABLE 1 Flash ignition delays and propagation modes. Associated TotalIgnition Delay (s) Error per % Continuous Igniter Setup Average Std DevTest (s) Ignition 1 nAl 0.33 0.18 0.01 20% 5 nAl 0.51 0.08 0.02 60% 1nAl × 1 μAl 0.47 0.09 0.03 70% 1 nAl × 3 μAl 1.04 0.30 0.03 90% 1nAl/μAl Mix 0.36 0.13 0.03 60% nAl/μAl Mix Film No Propellant Ignition 0% nAl Film No Propellant Ignition  0%

As nAl reacts rapidly, the soot formed locally around the aluminum isblown apart into smaller solid fragments, whereas μAl typically exhibitssoot formation in long, connected strands. As a protective glass slidehad to be placed close to the igniter composites 104, 106, there wasconcern that the glass may be inhibiting the propellant 108 byobstructing the release of soot from the reaction zone. To investigateif the protective slides were inhibiting the reactions and creating thedelayed ignition between the igniter composite 104, 106 and thepropellant 108, ignition delay tests were repeated with a laser setup102.

The laser 102 was capable of driving ignition at energies higher thanthe MIE of the Al/PVDF igniter composite 104, 106 materials. Forinstance, for nAl films, the MIE was found to be 0.7 J/cm2 when usingthe 1064 nm laser. The optical setup removed the necessity of aprotective glass slide altogether. If the delayed ignition in theflash-ignited propellants were solely caused by an inhibition ofreaction by the glass slide, the laser materials were hypothesized tohave no samples with delayed ignition. Some laser-ignited films such asthe 1 nAl×1 μAl, the 1 nAl×3 μAl, and the 75 wt. % nAl printed ignitershad more samples with continuous ignition, as shown in FIG. 19 , whencompared to the flash-ignited samples, as shown in FIG. 18 . However,other laser-ignited films such as the 1 nAl and 5 nAl printed ignitersdid not show enhanced continuous ignition, indicating that the glassslide was not responsible for the delayed ignition. Rather, it isbelieved this is due to the higher energies used to initiate thereaction as the energy deposited by the flash lamps was capped at 7J/cm2.

Unlike flash ignition, the laser light is focused onto the surface andminimal intensity was reflected into the camera. Therefore, as shown inFIGS. 20-22 , the first peak indicates the ignition of the ignitercomposite 104, 106 material. After the consumption of the ignitercomposite 104, 106 material, the intensity traces of the laser-ignitedsamples followed similar trends to the flash-ignited samples withcontinuous ignition indicated by steady intensity values after thealuminized igniter peak, as shown in FIG. 20 . The delayed ignition isrepresented by a decrease and then recovery of intensity, as shown inFIG. 21 . The extinguishment is illustrated by a fall to zero, as shownin FIG. 22 .

As shown in Table 2, the 1 nAl×1 μAl samples had the lowest totalignition delay of 0.72 seconds for the most consistent transitionsbetween the igniter composite 104, 106 and the propellant 108 oflaser-ignited samples. Primarily due to the number of samples withdelayed ignition increasing the average total ignition delay time, thelaser-ignited 5 nAl samples had shorter delay times than the 1 nAldespite the fewer number of layers.

TABLE 2 Laser ignition delays and propagation modes. Associated TotalIgnition Delay (s) Error per % Continuous Igniter Setup Average Std DevTest (s) Ignition 1 nAl 0.84 0.12 0.02 10% 5 nAl 0.61 0.05 0.02 50% 1nAl × 1 μAl 0.72 0.10 0.02 100%  1 nAl × 3 μAl 1.32 0.13 0.03 100%  1nAl/μAl Mix 0.54 0.05 0.02 90% nAl/μAl Mix Film No Propellant Ignition 0% nAl Film No Propellant Ignition  0%

A single layer of 75 wt. % nAl was also found to exhibit continuousignition in 90% of the laser-ignited samples, as compared to 60% offlash-ignited samples. Although the Al/PVDF formulation may be capableof propagating at a rate similar to formulations with only nAl, the sizeof the Al fuel particles has a critical role in transferring energy fromthe reaction of the igniter composite 104, 106 to the propellant 108 toproduce a continuous ignition. Further, the manner of ignition does notdepend solely on slowing down the propagation of the reaction to producea continuous ignition.

Tests of minimum ignition energy showed a minimum nAl content, such as acritical mass fraction of 0.375, was necessary for initiation of Al/PVDFfilms of 20 wt. % fuel content. As nAl content is increased, the minimumignition energy approaches an asymptote. Printed igniter systems 100 mayachieve ignition at lower energies due to their increased surfaceroughness of high amplitude, low frequency surface deviations thatworked as a light trap, absorbing more otherwise reflected light, whilealso forming hotspots.

Films of 20-30 μm thick nAl were unable to ignite AP compositepropellant 108 pellets due to their rapid consumption and poor transferof energy into the underlying propellant 108. Printed igniter systems100 of ˜125 μm and greater thicknesses ignited propellant 108 pellets;however, many exhibited poor energy transfer to the propellant 108pellets, resulting in delayed ignition of the propellant 108 beforerecovering to a steady reaction across the entire propellant 108.

Despite its fast consumption, thin layers of nAl material propagatedreadily to layers of μAl, but often resulted in delayed ignition intothe AP composite propellant 108, marked by a dimming of the flame. Toget a smoother, more continuous ignition between the igniter composite104, 106 to the propellant 108, a multi-layered igniter system 100 wasutilized. To flash ignite, the first composite layer 118 exposed to theflash bulb needed a critical mass fraction of nAl of around 0.375 toinitiate the reaction. Subsequent second composite layer(s) 120consisting of μAl slowed down the propagation rate and increased theheat transfer to the composite propellant 108. Advantageously, theslower propagation rate and increased heart transfer provided by thesubsequent second composite layer(s) 120 of μAl resulted in a steadier,continuous propagation from the igniter composite 104, 106 to thepropellant 108. Desirably, fewer μAl second composite layers 120 werenecessary when igniting the igniter system 100 with a laser drivensystem 102 at higher energies.

Another way to quantify the ignition delay is to look at the intensitycreated during the reaction. The samples dimmed during the ignitionperiod in between the nAl/PVDF igniter composite 104, 106 material andthe propellant 108. The trace of the light intensity captured in thehigh-speed video frames may be used to determine the location of thetransfer of the reaction from the igniter composite 104, 106 to thepropellant 108. The manner of transfer—continuous ignition, delayedignition, or extinguishment—can clearly be visualized using theintensity traces. As shown in FIGS. 14 and 21 , a dip occurred in theintensity traces between the consumption of the igniter composite 104,106 material and the propellant 108 before rising to an asymptoteindicating a steady state reaction of the propellant 108. The dipaligned with the visibly noticeable dimming in the corresponding imageframes, indicating a delayed ignition of the propellant 108. Delayedignition in both flash and laser ignited materials exhibited the sametrend. Continuous ignition was captured in the intensity traces by asteady intensity value following propellant 108 ignition, as shown inFIGS. 16 and 20 . Likewise, extinguishment occurred when the intensitytraces fell to zero after propellant 108 ignition, as shown in FIGS. 13and 22 .

In a specific instance, when igniting an igniter composite 104, 106sample with a broadband flash lamp, the bulb was exposed. This led to asaturation or near saturation of the field of view. More specifically,this exposure manifested in an initial spike of saturation as seen inFIGS. 13, 14, and 16 . In every ignition event with the flash lamp, theigniter composite 104, 106 began to react as the flash intensity wasdying down, making quantification of the delay between triggering theflash and the first light on the igniter composite 104, 106 materialdifficult through the use of intensity quantification alone. Ignition ofthe igniter composite 104, 106 was thus determined as the time at whichthe intensity had fallen to 10% of the highest value of the first peak.Similarly, the propellant 108 ignition would begin as the ignitercomposite 104, 106 material was consumed and was thus similarlydetermined at once the intensity had fallen to 10% of the highest valueof the second peak. After the second peak, the intensity trace wouldeither continue at a steady value which indicated continuous ignition,decrease in value before rising to an asymptotic steady state whichindicated a delayed ignition, or fall to zero which indicatedextinguishment.

Unlike flash ignition, laser ignition had a small spot size thatminimized intensity reflected into the camera. Therefore, as shown inFIGS. 20, 21, and 22 , the first peak was determined as the ignition ofthe igniter composite 104, 106 material, rather than an indicator of thelaser 102. After the consumption of the igniter composite 104, 106material, the intensity traces follow similar trends to the flashignited samples with continuous ignition indicated by steady intensityvalues, delayed ignition by a decrease and then recovery of intensity,and extinguishment by a fall to zero.

Advantageously, the igniter system 100 and methods 200, 300 may enhancethe reliability and the consistency of achieving a sustained ignition ofa propellant 108. Desirably, the use of a nano-scale aluminum firstcomposite layer 118 and micron-scale aluminum second composite layer 120provides a steadier and more continuous propagation. Where the ignitercomposite 104, 106 is provided as a first composite particle 122disposed within the propellant 108, the propellant 108 mayadvantageously be ignited directly from the optical energy source 102.

Example embodiments are provided so that this disclosure will bethorough and will fully convey the scope to those who are skilled in theart. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms, and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail. Equivalent changes, modifications and variations ofsome embodiments, materials, compositions, and methods can be madewithin the scope of the present technology, with substantially similarresults.

What is claimed is:
 1. An igniter system configured to be ignited by anoptical energy source emitting a wavelength between around 250nanometers to around 1100 nanometers, comprising: an igniter compositeincluding a reactive component and a fluoropolymer; a propellant coupledto the igniter composite; and wherein the igniter composite isconfigured to achieve a sustained ignition with the wavelength betweenaround 250 nanometers to around 1100 nanometers from the optical energysource.
 2. The igniter system of claim 1, wherein the reactive componentincludes at least one of a reactive metal and a metal oxide.
 3. Theigniter system of claim 2, wherein the reactive component is selectedfrom the group consisting of lithium, boron, sodium, magnesium,aluminum, silicon, calcium, titanium, vanadium, chromium, manganese,iron, cobalt, nickel, copper, zinc, gallium, yttrium, zirconium,niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium,indium, tin, antimony, iodine, cesium, barium, hafnium, tantalum,tungsten, platinum, gold, mercury, lead, bismuth, any oxides of themetals, and any combination thereof.
 4. The igniter system of claim 3,wherein the reactive component is aluminum.
 5. The igniter system ofclaim 1, wherein the fluoropolymer is selected from the group consistingof PVF (polyvinylfluoride), PVDF (polyvinylidene fluoride), PTFE(polytetrafluoroethylene), PCTFE (polychlorotrifluoroethylene), PFA(perfluoroalkoxy polymer), [P(VDF-TrFE)] (poly(vinylidenefluoride-trifluoroethylene)), [P(VDF-TrFE-CFE)] (poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene)), THV (a polymer oftetrafluoroethylene, hexafluoropropylene and vinylidene fluoride), FEP(fluorinated ethylene-propylene), ETFE(polyethylenetetrafluoroethylene), HTE (a polymer ofhexafluoropropylene, tetrafluoroethylene and ethylene), ECTFE(polyethylenechlorotrifluoroethylene), FFPM/FFKM (PerfluorinatedElastomer), FPM/FKM (Fluorocarbon [Chlorotrifluoroethylenevinylidenefluoride]), FEPM (Fluoroelastomer [Tetrafluoroethylene-Propylene]), PFPE(Perfluoropolyether), PFSA (Perfluorosulfonic acid), and any combinationthereof.
 6. The igniter system of claim 1, wherein the fluoropolymer isPVDF.
 7. The igniter system of claim 1, wherein the reactive componentincludes a first reactive component and a second reactive component, thefluoropolymer includes a first fluoropolymer and a second fluoropolymer,and the igniter composite includes: a first composite layer having afirst composite layer particle including the first reactive componentand the first fluoropolymer, wherein the first composite layer particleis a substantially nano-sized particle; and a second composite layerhaving second composite layer particle including the second reactivecomponent and the second fluoropolymer, wherein the second compositelayer particle is a substantially micro-sized particle.
 8. The ignitersystem of claim 7, wherein the second composite layer is disposedbetween the first composite layer and the propellant.
 9. The ignitersystem of claim 7, wherein the total thickness of one of the firstcomposite layer and the second composite layer is at least fivehundredths of a millimeter.
 10. The igniter system of claim 9, whereinthe total thickness of one of the first composite layer and the secondcomposite layer is between five hundredths of a millimeter to fivetenths of a millimeter.
 11. The igniter system of claim 1, wherein theigniter composite is constructed utilizing additive manufacturing. 12.The igniter system of claim 1, wherein the igniter composite isconstructed utilizing tape casting.
 13. The igniter system of claim 1,wherein the igniter composite is provided as nano-sized compositeparticle disposed in the propellant.
 14. The igniter system of claim 13,wherein the nano-sized composite particle includes thirty weight percentto fifty weight percent of the reactive component.
 15. The ignitersystem of claim 1, wherein the igniter composite is configured toachieve a sustained ignition with less than thirty joules per squarecentimeter of energy provided by the optical energy source.
 16. Theigniter system of claim 15, wherein the igniter composite is configuredto achieve a sustained ignition with less than five joules per squarecentimeter of energy provided by the optical energy source.